Organic EL elements have a basic construction comprising a substrate made of glass or the like, on which are stacked a first electrode, an organic functional layer including a light-emitting layer, and a second electrode.
Display devices which use organic EL elements have the following sorts of advantages over the liquid-crystal displays that currently represent the mainstream in flat panel displays.
1) They are self-emissive, so the viewing angle is wider. PA0 2) Displays only 2-3 mm thick can easily be manufactured. PA0 3) A polarizing plate is not used, so the color of the emitted light is natural. PA0 4) The broad dynamic range in brightness results in a crisp, vibrant display. PA0 5) They operate over a broad range in temperature. PA0 6) The response rate is more than three orders of magnitude faster than liquid-crystal displays, easily enabling the display of moving images. PA0 7) A very high luminance of from several 100 to about 1,000 cd/m.sup.2 can be obtained at a voltage of about 10 V. PA0 (1) An organic electroluminescent display device comprising a luminescent laminate and a terminal electrode on a substrate surface, wherein the luminescent laminate has, in order, a first electrode, an insulating layer covering part of the first electrode, an organic functional layer including a light-emitting layer, and a second electrode comprised of at least one conductive layer, and the at least one conductive layer comprising the second electrode is an interconnect layer which is in contact with at least part of the terminal electrode, and which has a slope at the end of not more than 0.1, at least in the vicinity of the terminal electrode. PA0 (2) The organic electroluminescent display device of (1) above, wherein the interconnect layer contains argon and the organic functional layer contains substantially no argon. PA0 (3) A method of manufacturing an organic electroluminescent display device comprising a luminescent laminate and a terminal electrode on a substrate surface, wherein the luminescent laminate has, in order, a first electrode, an insulating layer covering part of the first electrode, an organic functional layer including a light-emitting layer, and a second electrode comprised of at least one conductive layer, and the at least one conductive layer of the second electrode is an interconnect layer in contact with the terminal electrode, the method comprising the steps of forming a first electrode, an insulating layer, and a terminal electrode; subsequently placing the substrate so that the surface side faces a mask having a shielding portion for restricting the layer-forming region and an opening surrounded thereby; forming an organic functional layer; then forming an interconnect layer by a process having a better step coverage than the process used to form the organic functional layer such as to bring the interconnect layer into contact with at least part of the terminal electrode. PA0 (4) The method of manufacturing an organic electroluminescent display device of (3) above, wherein the substrate is placed on the mask in such a way that the shielding portion of the mask covers at least part of the terminal electrode across an intervening space. PA0 (5) The method of manufacturing an organic electroluminescent display device of (3) or (4) above, wherein the organic functional layer is formed by a vacuum evaporation process and the interconnect layer is formed by a sputtering process. PA0 (6) The method of manufacturing an organic electroluminescent display device of any one of (3) to (5) above, wherein the at least one conductive layer comprising the second electrode is made of a metal having a work function of not more than 4 eV.
However, there are number of difficult problems associated with the manufacture of EL elements, such as the ease with which the organic functional layer is degraded by moisture, oxygen and the like, the ready solubility of the organic functional layer in organic solvents, and the low resistance of this layer to heat. It is then a common practice to seal a luminescent laminate comprising a first electrode, an organic functional layer, and a second electrode with a sealing means composed of a stable material (e.g., a resin film, a metal film, an inorganic film, or a glass sheet).
In cases where organic EL elements are utilized in display devices, they generally have a construction in which a terminal electrode for connection to an external circuit is provided outside of the sealing means, and the second electrode and the terminal electrode are connected.
A method known as "masked film formation" is generally used to lead the second electrode out of the sealing means. Masked film formation is a method involving the steps of placing a mask having a shielding portion for restricting the film-forming region on a substrate or device and effecting deposition, thereby forming a film on the desired region of the substrate.
Using this method, the second electrode and the terminal electrode can be connected by the following procedure. First, an organic functional layer is formed on the substrate using a mask having an opening which corresponds to the organic functional layer-forming region. This is done in such a way that at least part of the terminal electrode is covered by the shielding portion of the mask. Next, before forming the second electrode, this mask is exchanged for a mask having an opening which corresponds to the second electrode-forming region (i.e., a mask having an opening larger than the mask used to form the organic functional layer), and the second electrode is formed. The second electrode thus formed extends beyond the organic functional layer-forming region. In this way, connection of the second electrode with the terminal electrode is accomplished.
However, the following problems are associated with the process of changing the mask following formation of the organic functional layer.
Because the mask is generally changed manually, this is done under normal atmospheric conditions. For example, in a case where the second electrode is itself used as the interconnect layer, after forming the organic functional layer, the substrate is returned to atmospheric conditions, the mask is changed, then the substrate is returned once again to the vacuum film-forming apparatus whereupon the second electrode is formed. Since this process allows moisture to be adsorbed onto the surface of the organic functional layer that is exposed to the atmosphere or be taken up at the interior of the layer, a decline occurs in the adhesion or electrical connection at the interface between the second electrode and the organic functional layer, necessitating a higher driving voltage when inducing light emission. Moreover, dark spots arise. These are defects involving an increase in the non-emissive regions at the edges of and within the light-emitting area. Another major drawback of this process is that foreign particles such as dust tend to settle on the organic functional layer, which also results in the formation of dark spots.
Although it is possible to change the mask using a robot or the like so as not to break the vacuum, the positioning mechanism in a vacuum is very large and elaborate, considerably increasing the cost of the film-forming apparatus. If such a positioning mechanism is not used, misalignment occurs, making it necessary to provide a wide margin. To the user of the display device, this appears as an area which does not luminesce, so that the region which could indeed be regarded as unnecessary becomes quite large. Moreover, in cases where a plurality of display devices are built onto a single substrate, providing a wide margin directly leads to a decline in the number of available devices, as a result of which the production cost per display device rises.
Another approach involves mounting the mask in the film-forming apparatus, but here too a large margin is required. Moreover, the mask is used over the course of many film-forming operations, dust particles are very often generated. These become major factors in lowering yields. This is no question that using a mask having a consistently high degree of cleanliness is desirable for improving yield.
However, it is customary to use the second electrode in an organic EL element as the cathode for injecting electrons. Because the cathode contains a metal having a low work function, it is highly susceptible to deterioration or corrosion by moisture, oxygen, and the like. Hence, it is preferable to use as the second electrode a laminate comprising an electrode layer of a low work function metal and a conductive layer of a more stable metal thereon.
It is sometimes desired to connect the second electrode with an external circuit by such means as wire bonding or press bonding using a heat seal connector or flexible printed circuit (FPC) connector. However, almost all such methods require mechanical strength and heat resistance at the connection between the second electrode and the terminal electrode. If the second electrode has a laminated structure comprising a corrodable electrode layer and a stable conductive layer, any damage to the conductive layer exposes the electrode layer and facilitates corrosion of the electrode layer. It is then desirable that an interconnect portion of the second electrode which extends to the terminal electrode be formed entirely from the stable conductive layer.
In cases where a second electrode having the above laminated structure is formed using the masked film formation process mentioned earlier, forming the respective layers by the method described below is advantageous for protecting the organic functional layer and the corrodable electrode layer from oxygen and moisture. In this method, first the mask is set in place and the organic functional layer, the corrodable electrode layer, and the stable conductive layer are successively formed. Next, the vacuum is broken and the mask is replaced with a mask having a larger opening, following which the vacuum is restored and a stable conductive layer is additionally laminated. Defects do not readily arise with this method because only the uppermost, conductive layer is connected to the terminal electrode, and neither the organic functional layer nor the corrodable electrode layer come into direct contact with the atmosphere. However, this approach does require that the vacuum first be returned to atmospheric pressure then later restored, and also that a stable conductive layer be formed twice, thus increasing the length of the manufacturing process and raising production costs.